Alkylation process



Feb. 2, 1965 p. K. BEAVON ETAL 3,163,591

ALKYLATION PROCESS Original Filed May 11, 1959 2 Sheets-Sheet l Feb. 2, 1965 a. K. BEAVON ETAL ALKYLATION PROCESS 2 Sheets-Sheet 2 Original Filed May 11, 1959 N Th MIQQQQ United States Patent 3,168,591 ALKYLATIUN PROCESS David K. Beavon, Loclrport, Ill, and Frank A. Clauson, Haddonfield, N..I.,assignors to Texaco Inc, New York, N.Y., a corporation of Delaware Continuation of application Ser. No. 812,492, May 11, 1959. This application Nov. 9, 1962, Ser. No. 238,318 5 Claims. (ill. zoo-seam This invention relates to an improved process for catalytic alkylation, and more specifically to such process wherein an olefin-based alkylatable material is alkylated with isobutane in the presence of a non-volatile alkylation Catalyst. This application is a continuation of the copending application of David K. Beavon and Frank A. Clauson, Serial No. 812,492 filed May 11, 1959, now abandoned.

The alkylatable material for reacting with isobutane is olefin-based, i.e. it is generally an olefinic hydrocarbon itself such as propylene, butylene, amylene or the like, but it also can be an alkyl sulfate (as obtained for example in a so-called two stage process wherein an olefinic hydrocarbon is absorbed in sulfuric acid as a first stage in the alkylation operation).

Advantages of. the instant process over conventional isobutane-olefin alkylation processes include these: a simplicity of product recovery and economy of returning unreacted isobutane to the alkylation; self-refrigeration coupled with the foregoing; and the ability to handle propane diluent effectively and economically without having to admit it to the alkylate recovery system to any significant extent whatsoever. The latter advantage is particularly important in the alkylation of a propylene-containing hydrocarbon feed because these feeds ordinarily contain a great deal of diluent propane with them, and it is diflicult to separate propane from propylene by conventional distillation techniques.

The instant catalytic alkylation process is conducted in an alkylation zone wherein the reactants, isobutane and olefin-based alkylatable material, are reacted in the liquid phase in the presence of a non-volatile alkylation catalyst under alkylating conditions; isobutane is the preponderant hydrocarbon in the reaction mixture, and. the materials fed to said alkylation zone contain in the aggregate at least about 0.3 mol percent of propane, this percentage being based on the total amount of fresh and recycled materials other than catalyst being fed to the alkylation zone.

Specifically in such system the improvement comprises: evaporating lower boiling components including part of the isobutane and virtually all of the propane and lighter hydrocarbons from the allrylation reaction mixture under conditions effective for refrigerating the alkylation zone, thereby providing a depropanized body of remaining liquid hydrocarbon and an isobutane-rich vapor containing propane, separating propane from said isobutane-rich vapor containing propane, discharging separated propane from the system, returning at least a portion of the resulting depropanized isobutane from said propane separation to said alkylation reaction zone, distillatively stripping butanes fromsaid remaining liquid hydrocarbon in the absence of substantial isobutane enrichment of the distillation, condensing the resulting stripped butanes distillate, and returning said stripped butanes distillate to said alkylation zone.

The drawings are flow sheets showing ways in which the instant process can be practiced, but they should not be construed as limiting the invention. They will be described more fully hereinafter.

By a non-volatile liquid allcylation catalyst there is meant, for example, sulfuric acid of at least about 88% strength, an aluminum chloride-hydrocarbon complex 3,168,591 Patented Feb. 2, 1965 (titratable acidity, weight percent) by the addition of make-up 98-99.5% sulfuric acid in amount sufficient to maintain this strength while purging some of the used acid from the system.

In the alkylation zone the preponderan-t hydrocarbon present is isobutane. At least about 55 volume percent and broadly about 60 to 90 volume percent of the entire hydrocarbon fraction of the reaction mixture is isobutane. This assists in directing the reaction towards the produc-' tion of the most valuable type of gasoline engine fuels. Generally the isobutane concentration is measured for convenience on the output flow from the reactor (or reactors as the case may be). This generally will be between 65 and 80 percent, but it can be even higher.

When contacting the excess isobutane with the alkylatable material and catalyst in the liquid phase alkylation temperatures substantially above about F. are usually not used. The most desirable low temperatures, eg ordinarily below about 75 F. and advantageously 30 to 55 F., can be maintained in the alkylation zone by selfrefrigeration of the alkylation reaction mixture. In my process such self-refrigeration not only is used to assist in controlling temperature of the reaction at a desired low value, but also it virtually precludes the admittance of propane intothe alkylation recoverydistillation system, and it also lifts a great deal of the isobutane recovery load from the fractional distillation alkylate recovery equipment with consequent saying of fixed. charges and utilities expenses for that part of the system.

In the self-refrigerated systems evaporation of lower boiling components, that is isobutane and more volatile materials present in the reaction mixture, whether said evaporation is done in the alkylation zone or apart therefrom, is broadly the technique used to refrigerate the reaction mixture (the cooling can be either direct or indiroot). In one form of self-refrigeration such lower boiling hydrocarbon components in the reaction mixture, including some of the isobutane present and virtually all of the propane, are evaporated directly from the reaction zone under a low pressure, e.g. 0 to 30 p.s.i.g., to

' comitant cooling of the remaining liquid hydrocarbons including alkylate and some unreacted isobutane. At least a part of said chilled remaining liquid. hydrocarbons are used to refrigerate the reaction zone indirectly through heat exchanger walls. In such operation the alkylation Zone and efiiuent separator are maintained under suffic'ient pressure to keep virtually all components present in the liquid phase (with the significant exception of, for example, possible cavitation around the propeller or other mixing device). Flashing in a flash zone as referred to herein denotes the practically adiabatic forming of chilled vapors and residual liquid by reduction of pressure on a liquid phase hydrocarbon-containing material having one or more components volatile at the reduced pressure.

. Other alkylation conditions include the use of a mol ratio of isobutane to olefin supplied to the alkylation zone (including isobutane recycle) substantially in excess of 1:1, and generally between about 4:1 and about 15:1; use of a liquid catalyst: liquid hydrocarbon volume ratio between about 0.5 :l and 5:1, and preferably about 1:1;

and use of an average holding time of reactants in the alkylation zone of between about a minute and about 45 minutes.

Because the instant process handles hydrocarbon feeds containing propane so elficiently and easily, it is not necessary to depropanize these process feeds extensively, i.e. to provide a supply having below about 0.3 mol percent (catalyst-free basis) to the alkylation zone. In some cases the propane present may reach as high as 15 volume percent of the hydrocarbons fed to the alkylation zone a typical propane-rich operation being one with a propylone feed containing propane and lighter hydrocarbons such as ethane and ethylene. Usually the propane present in the alkylation zone is between 0.5 and 5 volume percent in our process.

The propane and other light ends collect in the evaporated hydrocarbons resulting from refrigerating the alkylation zone. Usually these evaporated hydrocarbons are compressed sufliciently high, e.g. 40 to 130 p.s.i.g., to be condensed indirectly with the cooling Water available at the refinery. The condensate, however, can be formed by subcooling with indirect ammonia or Freon refrigeration or the like, if desired. The condensate is partially or completely depropanized, suitably by conven tional fractional distillation using the ordinary fractional distilling column (because the boiling points of propane and isobutane are quite widely divergent).

The depropanizing can be operated simply on a bleed stream of the total condensate to keep propane from building up in the alkylation zone (where it is an undesirable diluent deleterious to best operation). Alternatively, the entire isobutane-rich vapor containing propane can be depropanized, suitably by fractional distillation.

Using either depropanizing technique there results a body of isobutane-rich liquid (depropanizer bottoms with or without straight condensate) which can be sent back to the alkylation zone to assist in maintaining the de sirable high concentration of isobutane in the reaction mixture. Advantageously, however, this isobutane-rich liquid body is subjected to a reduction in pressure whereby it refrigerates itself with the concomitant generation of vapors and chilled residue liquid. The so-generated vapors are passed into admixture with the evaporated low boiling components being fed to the compressing and condensing system before depropanizing, and the chilled isobutane-rich residue liquid is sent back to the alkylation reaction zone where it not only assists in maintaining a high concentration of isobutane therein, but also provides a measure of direct refrigeration for said zone.

In a still another depropanizing alternative molecular sieves can be used to remove part or all of the propane and other straight chain hydrocarbon diluents from the evaporated low boiling components. Said low boilers can be in the vapor or in the liquid phase for such depropanizing. Such molecular sieves suitably are zeolitic minerals with a multitude of substantially uniform sorptive pores having an effective size or diameter of about 4 to 6 A. A particularly efficient zeolite of this type can be made from Type A zeolite properly base-exchanged. Type A zeolite is a synthetic material not found in nature, and it is described in the articles by Breck et al. and Reed et al. in the Journal of the American Chemical Society, volume 78, No. 23 of December 8, 1956, from pages 5963-5977.

A significant feature of the instant process is that as much as half of the isobutane recovery load or even more is diverted from the alkylate distillative processing equipment and directed toward the depropanizing step as hereinbefore described.

The remaining liquid hydrocarbon after the evaporative refrigerating of the alkylation zone, that is the liquid hydrocarbon fraction of the reaction mixture from an autorefrigeration system or the unevaporated efiluent from an efliuent refrigeration system, contains the product alkylate, and it must be further processed for recovery thereof. In the case of the autorefrigeration system the unevaporated hydrocarbon portion is in emulsion with catalyst; this is settled out to leave a hydrocarbon phase discharging from the settling zone. In the eflluent re frigeration system, however, catalyst separation has preceded evaporation of hydrocarbons for refrigerated purposes.

Said remaining liquid hydrocarbon in either system is then suitably rid of traces of acid, ordinarily by the conventional technique of Washing with caustic soda or the like then with water, typically at an elevated temperature of about 60120 F. and under suflicient pressure to maintain the hydrocarbons in the liquid phase. Alternatively, the treatment can include or be exclusively a treatment with an adsorbent such as activated alumina, bauxite, or the like to prepare the hydrocarbon for the distillative recovery processing.

The neutralized or otherwise treated liquid hydrocarbon preferably is sent to a stripping column, that is a conventional fractional distillation tower which has a suitable heat source at the bottom and no external reflux to the top of the column (the feed being the reflux). The distillate from such stripping operation is mixed butanes. It is slightly richer in isobutane than the tower feed. Such butanes distillates is condensed and returned to the alkylation reaction zone, suitably with intermediate indirect cooling.

In one advantageous instance a single butanes stripping column can be used for the preparation of motor fuel; one or more side taps then are used for removing the normal butane concentrate, if desired, and the bottoms of the column are withdrawn as alkylate for use in blending of motor fuel stocks. Alternatively, additional fractionating columns can be used to debutanize the alkylate product and to rerun it to maintain it within a narrow boiling point range for aviation alkylate or the like, aviation alkylate having end boiling point not above 338 F. In yet onother alternative part or all the C and lighter hydrocarbons can be topped from neutralized alkylate-containing hydrocarbon liquid in a first distillation, and these topped C and lighter substances subjected to a second distillation which is done with little or no external reflux, i.e. a stripping. The resulting distillate is recycled to the reaction zone, while the bottoms from the first distillation can constitute the product alkylate.

The absence of substantial isobutane enrichment of the distillation in the alkylate recovery processing of the instant process provides a way of separating butanes from the alkylate with comparatively low expense for utilities, that is steam for reboiling and cooling water for condensing reflux. If desired, a comparatively small amount of isobutane enrichment (rectification) can be used above the feed inlet of the butane stripping column, e.g. by use of reflux ratio of /2 to 1 part of distillate being returned as external reflux per part being withdrawn as overhead product. Such slight butane enrichment, while not substantial, can be of particular value when the make-up isobutanes supply or the stripping column contains a substantial proportion of normal butane (as could be obtained for example, from the BB stream of a thermal cracking unit).

The reactor or reactors (in series or in parallel arrangement) used can be of a conventional type, e.g. one employing internal recirculation such as the sooalled Stratco Contactor; one employing autorefrigeration such as the so-oalled Cascade reactor; a pumpand-time-tank type unit wherein the average time of contact of the alkylation reaction mixture and catalyst is generally between about 5 and about 45 minutes and, advantageously, is 5 to 20 minutes; one employing external recycle of alkylation emulsion; or one employing injection of reactor feed for agitation of the reaction mixture. It is usually advantageous to agitate the alkylation reaction mechanically, but this can be dispensed with or minimized when using some conventional reactor types.

When propylene is being alkylated, it is advantageous to promote the alkylation with 5 to 20 liquid volume percent of butylene or other higher olefin, said percentages being based upon total olefin fed to the alkylation zone as olefin itself or other'olefin-based material such as alkyl sulfate. When butylenes are being ialkylated, a suitable feed is a conventional B-B feed from a thermal or a catalytic cracking unit.

Reference is made to FIG. 1. It is the flow sheet of a refinery alkylation unit. The operation described was carried out by alkylating butylenes with isobutane. All flows are given in 42 gallon barrels per day. Analyses are given in liquid volume percent. The essential equipment is depicted in conventional fashion. Most of the equipment items are numbered, the exceptions being LLC referring to liquid level controls actuating certain unnumbered throttling valves in lines hereinafter described, and reboilers, which are only shown symbolically at the base of the fractional distillation towers. Sulfuric acid consumption over a'sustained period of time, of which the operation described was a typical operating day, was 0.303 lb. per gallon of alkylate.

BB charge was passed into the system through line 10, this charge constituting 2142 b.p.d. and analyzing 0.5 percent propane and lighter, 52.9 percent isobutane, 17.8 percent normal butane, 24.3 percent butylene, 3.3 percent isopentane and 1.2 percent pentylene. This fed and cooled stripped butane distillate from line 95, obtained as hereinafter described, were passed through line 12 and heat exchanger 13, then through line 14 and into water trap 15. Water that settled out in the chilled feeds was tapped off outlet 16.

Thus dried, the hydrocarbon passed through line 18 into contactor 21, which constitutes the alkylation reaction zone. Also flowing into line 18 was -a chilled, depropanized isobutane-rich residue liquid entering from line 17, said residue being 2304 b.p.d. containing 0.1% propane and lighter, 80.1% isobutane, 18.1% n-butane, and 1.7% isopentane.

Contactor 21 was equipped with an agitator, internal cooling coils 32, and head 30. The volume percentage of sulfuric acid in the reaction mixture (emulsion) was about 50, and the emulsion temperature was 44 F. Total hydrocarbon charged to the container was 7.322 b.p.d., and .it was composed of 0.5% propane, 71.0% isobutane, 18.8% normal butane, 7.1% butylene, 2.2% isopentane, and 0.4% pentylene.

The emulsion discharged continuously through line 22 into acid settler 23. The hydrocarbon fraction of this output efiluent (reacted mix) contained 0.5 propane and lighter constituents, 67.7% isobutane, 18.3% normal butane, 2.7% isopentane, and 10.8% hexane and higher molecular weight hydrocarbons.

The flat sulfuric acid catalyst that settled in settler 23 was withdrawn through line 24 and returned to the contactor through line 20. A purge of spent acid was removed from the system by means of outlet 25, and a make-up of fresh high strength sulfuric acid substantially equivalent in volume thereto entered line 26, the purge and make-up being regulated so as to maintain the sulfuric acid strength in the acid catalyst fraction of the reaction mixture at 93.5%.

The separated hydrocarbon phase of the efiluent was withdrawn through line 27 and passed through back pressure control valve 28 wherein the pressure was reduced from about 40 p.s.i.g. to approximately 16 p.s.i.a. withthe concomitant formation of a chilled residual liquid and a vapor phase of part of the isobutane present and virtually all of the propane and lighter hydrocarbons in the reacted mix. This cold vapor-liquid mixture (about 28 F.) passed through inlet 29, into head 30, through cooling coil 32 (wherein further evaporation of liquid hydrocarbons took place), through outlet 33, and into vapor-liquid separator 34. l

The vapor phase resulting from the foregoing refrigeration operation was disengaged from hydrocarbon liquid in vessel 34 and passed through lines 35 and inlet 36 into drum 37. Trapped liquid can be drained from outlet 38. The vapors continued through line 39, were com pressed in compressor 40, discharged through watercooled condenser 43, and therein condensed. The condensate passed through line 44 into vessel 45. Also fed in with the vapors from the refrigeration of the reactor were vapors from the flashing of depropanizer bottoms, obtained as hereinafter described, these vapors entering by means of line 65.

Pump 46 delivered the condensed isobutane-rich vapors through pipe 47, feed-bottoms exchanger 48, then through line 49, into depropanizer 50, a fractional distillation column operated in conventional manner. 4

An overhead distillate passed from depropanizer 50 through condenser 52 and into drum 55. Pump 57 returned liquid distillate through line 58 as reflux to the depropanizer. From outlet 56 was withdrawn a propanerich distillate as L.P.G. to tankage at the rate of 24 b.p.d., the analysis being 96.2% propane and lighter, and 3.8% isobutane.

Depropanizer bottoms, rich in isobutane and virtually denuded of propane, were passed through line 59, feedbottoms heat exchanger 48, line 60, water cooler 62, and throttling valve 63. The resulting adiabatically chilled liquid-vapor mixture from this pressure reduction was discharged into vessel 64. The hydrocarbon vapors so generated were disengaged from the chilled hydrocarbon liquid and were sent through line 65 into compression with the light hydrocarbon vapors leaving vapor-liquid separator 34 as hereinbefore described.

Pressure in vessel 64 was about that of the compressor suction. The chilled, depropanized, isobutane-rich liquid residue in vessel 64 passed through line 66, pump 67, line 17, and line 18 back to alkylation contactor 21 to assist in maintaining the low temperature therein and a high fraction of isobutane in the reaction mixture.

The remaining liquid hydrocarbons disengaged in vaporliquid separator 34 passed through line 68 and pump 69, then discharged through line 78, heat exchanger 13,. and on through line 72, line mixer 73, and into neutralizer settling tank 74. Aqueous caustic soda which settled out in tank 74 was recirculated by pump 77 and line 78 into intimate contact with the hydrocarbon entering this conventional neutralizing apparatus. Caustic soda solution purge was maintained through line '75, and makeup was through line 76.

The neutralized liquid hydrocarbon passed from vessel. 74 through line 79 and into line mixer 82 together with a flow of wash water entering the system through line 80. The washed mixture passed through line 83 and into settler 84, Water being discharged through outlet 85 and washed hydrocarbon through line 86.

The hydrocarbon in line 86 entered stripping tower 87, a 60-tray tower, above the top tray. There was no reflux induced by external condensation on stripper 87.

It was operated to provide stripped butanes as distillate which passed through line 88 and condenser 89 at the rate of 3706 b.p.d. The distillate composition was 0.7% propane and lighter constituents, 77.2% isobutane, 20% normal butane, 0.1% butylene, 1.9% isopentane, and 0.1% pentylene. tankage for storage by means of outlet 92, the balance of 2876 b.p.d. was runthrough line 93, cooler 94, and line 95, then to the alkylation zone as hereinbefore described.

Additional make-up hydrocarbons were fed to stripper 87 at a point about 25 trays up from the bottom thereof to supplement the isobutane being fed to the system. This feed inlet is not shown. Said make-up flow was 552 b.p.d., and it analyzed 3.4 percent propane and lighter,

Of this distillate 830 b.p.d. passed to 0.2 percent propylene, 33.8 percent isobutane, 55.1 percent normal butane, 2.7 percent butylene, 3.4 percent isopentane, 0.7 percent n-pentane, and 0.7 percent pentylene.

The stripper was operated in conventional fashion to yield a bottoms fraction, which was withdrawn through line 96, said bottoms fraction being substantially denuded of isobutane and hydrocarbons having a boiling point lower than isobutane. This bottoms material was fed to product debutanizer 07, a fractional distillation tower operated in conventional fashion. From the product debutanizer an overhead product fraction consisting of 497 b.p.d. of a hydrocarbon mixture of 1.7% isobutane, 94.9% normal butane, 1.9% butylene, 1.3% isopentane and 0.2% pentylene was withdrawn by means of line 101. The overhead distillate in this case was condensed in condenser 98 and passed into tank 99, and the reflux was returned to tower 97 by means of line 100.

The debutanizer bottoms fraction (1003 b.p.d. having an A.P.I. gravity of 70.10, initial boiling point of 107 F., and end boiling point of 374 F.) passed through line 102, into heat exchanger 103, then to final fractionator 104 where it was fractionally distilled into a light alkylate and alkylate bottoms in conventional manner. Exhaust steam was injected into the reboiler through line 71 to assist in fractionation. The distillate was condensed in condenser 105, passed to tank 106, part was pumped through line 107 to reflux tower 104, and the balance (950 b.p.d. having an A.P.I. gravity of 71.1, an initial boiling point of 106 F., and an end boiling point of 288 F.) was removed from the system through line 108.

The alkylate bottoms were withdrawn from tower 104 by means of line 109, pumped through cooler 81, and discharged to tankage through line 110.

Reference is made now to FIG. 2. Olefin feed is charged to the system through line 111, is chilled in coils 112, and is discharged through line 114. Reaction zone 116 shown in FIG. 2 typifies a conventional Cascade type alkylation reactor which is divided into a plurality of zones by baffles 117 and 118, the reaction being conducted in the five agitated zones indicated, the agitators being represented by items 119. To the left of the agitated zones is an isobutane and recycle sulfuric acid receiving zone, termed a preflash zone, and to the right of the agitated zones is space for settling out sulfuric acid catalyst from crude alkylate. Drawotf pipe 120, internal in reactor 116, accepts the settled crude alkylate hydrocarbon phase to the exclusion of sulfuric acid catalyst.

The chilled olefin feed from line 114 is divided and a portion sent to each baffled reaction zone in reactor 116 by means of inlets 115. Also feeding vessel 116 is an isobutane-rich liquid obtained from depropanizer bottoms and entering vessel 116 by means of pipe 141, plus stripped butane distillate and recycle sulfuric acid catalyst entering through header 133. Series flow of reaction mixture is maintained from left to right in vessel 116 withthe emulsion eventually settling into an acid phase and a hydrocarbon phase in the right-hand portion of the vessel.

Pressure on the reactor is maintained at about 5 p.s.i.g. whereby evaporation of part of the isobutane and virtually all of the propane present occurs throughout the reactor to maintain desirable low alkylation temperature. The evaporated hydrocarbons are collected in header 131, then passed back into vessel 116 and out line 122. They are m'med with vapors in line 157 from the flash refrigeration of depropanizer bottoms and passed through line 134 for compression by compressor 135.

Settled sulfuric acid catalyst is returned from the settling portion of reactor 116 to the preflash zone by means of lines 121 and 133. The catalyst is maintained at alkylation strength by the purging of a portion and the making up thereof with fresh high strength sulfuric acid.

The compressed light ends are withdrawn through line 136 passed into condenser 137. The resulting condensate flows through line 138 and discharges into tank 139. Condensate is withdrawn from tank 139 by means of line 140. A fraction of condensate is shunted through line 142, pressure reducing valve 155, and line 156. The pressure reduction occurring causes adiabatic cooling of the condensate with the resulting generation of hydrocarbon vapors and a chilled residue of isobutane-rich liquid. Thus vapor-liquid mixture is passed through line 156 into heat exchanger 113. The vapors are disengaged therein and fed through line 157 as hereinbefore described. The chilled isobutane-rich liquid residue is conducted to vessel 116 by means of inlet 141 as hereinbefore described.

A bleed stream of the condensate is passed (at a rate suflicient to keep propane from building up in the system) through line 143 into depropanizer 144, a fractional distillation column operated in conventional manner to give a substantially depropanized bottoms fraction and an overhead distillate rich in propane.

The distillate passes through line 145, condenser 14-6, line 147, and into tank 148. Propane is withdrawn from the system through line 149. The balance of the condensate is passed through line 150, pump 152, and reflux line 153 for return to the depropanizer. The liquid bottoms fraction from the depropanizer is Withdrawn through line 154 and passed through pressure reducing valve 155 for adiabatic flashing and self-refrigeration together with the condensate entering said valve through line 142 as hereinbefore described.

Crude alkylate hydrocarbon of the reactor passes through line 123 and is treated for removal of acid traces and the like in treating section 124 (which can be, for example, a caustic soda neutralizing and water washing apparatus as hereinbefore described). drocarbon passes through line and into the top of stripping column 126.

Stripping column 126 is operated in the absence of any substantial isobutane enrichment from external condensation (external reflux). It is operated to separate virtually all of the isobutane and such normal butane as is vaporized therewith from the alkylate product. Butanes distillate passes through line and is condensed in condenser 132, then recycled to alkylation reactor 116 by means of line 133.

Make-up butanes, a mixture containing iso and normal butanes are fed into the system by means of line 127. They enter the stripping column at an intermediate point for distillation overhead of isobutane content. Line 128 is a withdrawal point for normal butane-rich liquid. At this point one can withdraw trapped liquid from the column and flash 01f butanes therefrom to prevent an undesirably high quantity of normal butane appearing in the bottoms product. Bottoms product from the stripping operation is withdrawn through 129. It is an alkylaterich fraction, and it is suitable for use in motor fuel without further treatment. It can, however, be subjected to a rerun finishing distillation, if desired.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. In a catalytic alkylation process wherein isobutane and olefin-based alkylatable material are reacted in an alkylation zone in the liquid phase in the presence of a non-volatile alkylation catalyst under alkylating conditrons, isobutane is the preponderant hydrocarbon in the reaction mixture, and the materials fed to said alkylation zone contain propane and normal butane diluents, the improvement which comprises:

(1) evaporating lower boiling components including part of the isobutane and virtually all of the propane and lighter hydrocarbons from the hydrocarbon portion of the alkylation reaction mixture under conditions effective for refrigerating the alkylation The neutralized hy- Zone, thereby providing a depropanized body of remaining liquid hydrocarbons and an isobutane-ricli vapor containing propane,

(2) condensing said isobutane rich vapor forming an isobutane-rich condensate,

(3) passing at least a portion of said isobutane-rich condensate to said alkylation zone,

(4) passing said remaining liquid into a distillation zone at the top of said distillation zone, said feed comprising reflux in said distillation zone,

(5) withdrawing distillate comprising isobutane from said distillation zone,

(6) passing at least a portion of said distillate to said alkylation zone,

(7) withdrawing a liquid stream comprising normal butane as a side stream from said distillation zone, and

(8) withdrawing debutanized liquid comprising pro uct alkylate as bottoms from said distillation zone.

2. The process of claim 1 wherein a hydrocarbon stream comprising isobutane and normal butane is introduced into said distillation Zone intermediate the point where said remaining liquid is passed into said distillation zone distillate is passed to said distillation zone as external reflux at a reflux ratio of less than one part of distillate being returned as external reflux per part being withdrawn as overhead product.

5. The process of claim 1 wherein said isobutane-rich condensate is depropanized and depropanized condensate is passed to said alkylation zone.

References (listed in the file of this patent UNITED STATES PATENTS 2,818,459 Gantt Dec. 31, 1957 2,828,348 Stiles et al Mar. 25, 1958 2,829,181 Stiles et a1. Apr. 1, 1958 OTHER REFERENCES Goldsby et al.: The Oil and Gas Journal, September 19, 1955, pp. 104-107. 

1. IN A CATALYTIC ALKYLATION PROCESS WHEREIN ISOBUTANE AND OLEFIN-BASED ALKYLATABLE MATERIAL ARE REACTED IN AN ALKYLATION ZONE IN THE LIQUID PHASE IN THE PRESENCE OF A NON-VOLATILE ALKYLATION CATALYST UNDER ALKYLATING CONDITIONS, ISOBUTANE IS THE PREPONDERANT HYDROCARBON IN THE REACTION MIXTURE, AND THE MATERIALS FED TO SAID ALKYLATION ZONE CONTAIN PROPANE AND NORMAL BUTANE DILUENTS, THE IMPROVEMENT WHICH COMPRISES: (1) EVAPORATING LOWER BOILING COMPONENTS INCLUDING PART OF THE ISOBUTANE AND VIRTUALLY ALL OF THE PROPANE AND LIGHTER HYDROCARBONS FROM THE HYDROCARBON PORTION OF THE ALKYLATION REACTION MIXTURE UNDER CONDITIONS EFFECTIVE FOR REFRIGERATING THE ALKYLATION ZONE, THEREBY PROVIDING A DEPROPANIZED BODY OF REMAINING LIQUID HYDROCARBONS AND AN ISOBUTANE-RICH VAPOR CONTAINING PROPANE, (2) CONDENSING SAID ISOBUTANE RICH VAPOR FORMING AN ISOBUTANE-RICH CONDENSATE, (3) PASSING AT LEAST A PORTION OF SAID ISOBUTANE-RICH CONDENSATE TO SAID ALKYLATION ZONE, (4) PASSING SAID REMAINING LIQUID INTO A DISTILLATION ZONE AT THE TOP OF SAID DISTILLATION ZONE, SAID FEED COMPRISING REFLUX IN SAID DISTILLATION ZONE, (5) WITHDRAWING DISTILLATE COMPRISING ISOBUTANE FROM SAID DISTILLATION ZONE, (6) PASSING AT LEAST A PORTION OF SAID DISTILLATE TO SAID ALKYLATION ZONE, (7) WITHDRAWING A LIQUID STREAM COMPRISING NORMAL BUTANE AS A SIDE STREAM FROM SAID DISTILLATION ZONE, AND (8) WITHDRAWING DEBUTANIZED LIQUID COMPRISING PRODUCT ALKYLATE AS BOTTOMS FROM SAID DISTILLATION ZONE. 